The frequency spectrum of a digital radio system is broken into channels that are small sub-spectrums. A first transmitter and receiver pair establishes a communication link over a first predetermined channel while other transmitter and receiver pairs use other predetermined channels. The transmitter transmits to the receiver over the channel using a predetermined data rate and modulation scheme (i.e., BPSK, QPSK).
Typically, a data transmission consists of two parts. The first part is a preamble that is relatively easy for the receiver to detect and to synchronize with. The preamble may be, for example, a period of unmodulated carrier signal or a period of carrier signal modulated by a known training sequence using a simple modulation scheme. The second part of a data transmission is a modulated waveform that contains the unknown information data bits that are being transmitted.
The data rate of the transmission is usually measured in bits per second (bps), including kilobits per second (Kbps) and megabits per second (Mbps). The number of bits per second is related to the type of signaling (also known as encoding and modulation) that is used to convey the information and the number of times per second that the transmitted signal changes its value. For example, in a frequency-shift-keyed (FSK) digital signal radio system, data is encoded by generating frequency deviations away from the carrier frequency. Decoding the transmitted information entails measuring the frequency deviations away from the carrier frequency and inferring the transmitted information.
However, if the transmitted carrier frequency is at a frequency other than the nominal frequency the receiver expects, the measurement of frequency deviation becomes inaccurate. Thus, the performance and sensitivity of the receiver are degraded. This is a known problem in FSK digital radio systems. The above-described problem is depicted in greater detail in FIGS. 1A through 2B.
FIG. 1A illustrates a frequency-shift keyed (FSK) carrier signal that is properly aligned to a receiver reference carrier signal. The transmitted carrier frequency is shown as a solid line and the receiver carrier frequency is shown as a dotted line. When no data bits are being transmitted, the transmitter carrier signal is equal to some center frequency value, such as 600 MHz. The receiver carrier reference signal is aligned with the center frequency value. For the sake of clarity, the dotted line representing the receiver carrier frequency is slightly offset in FIG. 1 from the solid line representing the transmitted carrier frequency so that the two lines do not coincide.
When data bits are transmitted, the frequency of the transmitted carrier signal is varied above and below the nominal or center frequency. These frequency variations are represented by the up and down arrows in FIG. 1A. For example, a Logic 1 may be transmitted by changing the transmitter frequency to 100 KHz above the center frequency and a Logic 0 may be transmitted by changing the transmitter frequency to 100 KHz below the center frequency. Thus, in the exemplary embodiment, a Logic 1 would be transmitted as 600.1 MHz and a Logic 0 would be transmitted as 599.9 MHz.
Within the receiver, the frequency variations in the transmitted carrier signal are translated into amplitude variations in the output voltage of a frequency discriminator or a similar circuit. FIG. 1B illustrates the amplitude modulated output of a frequency discriminator receiving an FSK carrier signal that is properly aligned with a reference voltage representing the receiver reference carrier signal. The amplitude modulated output voltage of the frequency discriminator is shown as a solid line and the reference voltage representing the receiver carrier frequency is shown as a dotted line. For the sake of clarity, the dotted line representing the amplitude modulated output voltage is slightly offset in FIG. 1B from the solid line representing the reference voltage so that the two lines do not coincide.
The amplitude modulated output voltage of the frequency discriminator is compared to the reference voltage to determine the value of the transmitted data. When no data bits are being transmitted, the amplitude modulated output voltage is equal to the reference voltage. When a Logic 1 data bit is transmitted and the transmitter frequency increases to, for example, 100 KHz above the center frequency, the frequency discriminator increases the amplitude modulated output voltage above the reference voltage. When a Logic 0 data bit is transmitted and the transmitter frequency decreases to, for example, 100 KHz below the center frequency, the frequency discriminator decreases the amplitude modulated output voltage below the reference voltage. A voltage comparator circuit translates the voltage differences into Logic 1 values and Logic 0 values. In the example shown in FIGS. 1A and 1B, the data sequence 101100 has been transmitted.
FIG. 2A illustrates a frequency-shift keyed (FSK) carrier signal that is not properly aligned to the receiver reference carrier signal. The transmitted carrier frequency has drifted to a higher center frequency than in FIGS. 1A and 1B. The transmitted carrier frequency is shown as a solid line and the receiver carrier frequency is shown as a dotted line. The receiver carrier reference frequency is so far below the new transmitted carrier frequency that positive and negative frequency variations of the transmitted carrier signal above and below the new center frequency are both higher than the receiver carrier frequency. Thus, positive and negative frequency variations are both represented by up arrows in FIG. 2A.
FIG. 2B illustrates the amplitude modulated output of a frequency discriminator receiving an FSK carrier signal that is misaligned with a reference voltage representing the receiver reference carrier signal. The amplitude modulated output voltage of the frequency discriminator is shown as a solid line and the reference voltage representing the receiver carrier frequency is shown as a dotted line. As a result of the increase in the transmitted carrier frequency, the receiver reference voltage is so far below the amplitude modulated output voltage of the frequency discriminator that positive and negative amplitude variations in the amplitude modulated output voltage are both higher than the reference voltage. As a result, comparison of the amplitude modulated output voltage and the reference voltage translates the voltage differences into inaccurate Logic 1 and Logic 0 values. In the example shown in FIGS. 2A and 2B, the transmitted data sequence is inaccurately determined to be 111111.
Therefore, there is a need in the art for improved frequency shift keyed (FSK) receivers that are capable of more accurately adjusting for frequency drift in either the incoming transmitted carrier frequency or the receiver carrier reference signal.